CRUSTACEA. 177 



simple membranous labrum above, by a bifid labium below, and by a 

 pair of maxillae and mandibles at the sides ; the mandibles are the 

 legs or jointed appendages of the fourth cephalic segment. In the 

 common crab you will observe behind and exterior to the mandiblps 

 a second pair of jaws, which, like the first, have their principal part 

 terminated by a cutting edge : the two following pairs of jaws are dis- 

 tinguished by their articulated feeler or palp, and by their flabelliform 

 appendages, which penetrate into the interior of the branchial cavity ; 

 the maxillary plate is armed with teeth : the fifth and last pair of 

 jaws has the maxillary plate so much expanded as to cover and protect 

 the whole oral apparatus : it has likewise a palp and flagellum articu- 

 lated to its base. All the foregoing maxillary organs are modifications 

 of entire limbs, which are thereby translated from the locomotive 

 series ; the jointed legs so metamorphosed belong to the last four 

 cephalic, and the first two thoracic, segments. 



The alimentary canal is most simple in the suctorial Crustacea, in 

 which it presents no noticeable difference from that in the Epizoa; 

 the hepatic appendages are however more localised and better de- 

 veloped. 



In the Limulus the mouth is situated nearly in the centre of the 

 inferior surface of the great cephalo-thoracic segment ; the oesopha- 

 gus is continued from it in a very unusual course forwards, and ex- 

 pands into a stomacli, which is situated at the anterior part of the 

 head. This organ is abruptly bent upon itself upwards and back- 

 wards, and is continued by a gradual diminution of diameter, as ap- 

 {)ears upon an external view, into the intestine, which passes back- 

 wards with a slight vertical bend, to the base of the penultimate ab- 

 dominal segment. When we examine the interior of the alimentary 

 tract, the distinction between the stomach and intestine is effected, as 

 Van der Hoeven has sliown, by a conical valvular pylorus, which 

 projects into the commencement of the intestine. The stomach is 

 lined by a very dense and corrugated horny membrane. The hepatic 

 mass, which, with the generative glands, fills the greater part of the 

 cephalo-thoracic cavity, pours its secretion into the commencement 

 of the intestine by two ducts on each side.* 



In the Stomapods or Squillas the stomach also bends forwards in 

 advance of the cardiac orifice ; a bi-articulate plate extends from 

 that orifice backwards, through the pylorus into the intestine, and 

 regulates the passage of the alimentary substances into that tube ; they 

 are previously subjected to the action of four lateral dentated pyloric 

 processes. 



In the higher Crustacea the stomach is of a globular form, more 

 * Piep. No. 477, A. 



